The specific aim of this multicenter study is to determine the location of a modifier gene. This will be accomplished by analyzing DNA from siblings with cystic fibrosis, their parents, and from individuals with cystic fibrosis who had meconium ileus at birth and their parents. A genetic modifier of cystic fibrosis in the mouse has been mapped to the proximal portion of mouse chromosome 17, a region syntenic with human chromosome 19q13. The cystic fibrosis modifier locus was mapped to the proximal region of mouse chromosome 7 through linkage studies with SFTR-deficient mice. Studies in humans have indicated that a cystic fibrosis modifier locus associated with meconium ileus, but not with pulmonary function abnormalities, does exist. To refine the mapping of the cystic fibrosis modifier locus, additional sibling pairs with cystic fibrosis were ascertained. A total of 161 cystic fibrosis families with at least two affected siblings (197 sibling pairs) have been collected from ten cystic fibrosis centers, including the University of Utah. This region will now be further refined by analyzing additional markers with testing for allelic associations with the disease locus. The Utah Cystic Fibrosis Center has contributed the majority of the ascertained subjects for this study. No results are yet available.